Shock absorbers are used in conjunction with automotive suspension systems to absorb unwanted vibrations which occur during driving. Shock absorbers are generally connected between the sprung portion (body) and the unsprung portion (wheels) of the automobile. A piston is located within a working chamber defined by a pressure tube of the shock absorber, with the piston being connected to the sprung portion of the automobile through a piston rod. The pressure tube is connected to the unsprung portion of the vehicle by one of the methods known in the art. Because the piston is able, through valving, to limit the flow of damping fluid between opposite sides of the piston when the shock absorber is compressed or extended, the shock absorber is able to produce a damping force which damps the unwanted vibration which would otherwise be transmitted from the unsprung portion to the sprung portion of the automobile.
Shock absorbers have been developed to provide different damping characteristics depending upon the speed or acceleration of the piston within the pressure tube. Because of the exponential relation between the pressure drop and flow rate, it is difficult to obtain a damping force at relatively low piston velocities, particularly at velocities near zero. Low speed damping force is important to vehicle handling since most vehicle handling events are controlled by low speed vehicle body velocities. It is also important to control damping force over the broad range of pressures generated across the piston as the piston velocity increases.
Various prior art systems for tuning shock absorbers during low speed movement of the piston use a fixed low speed bleed orifice to provide a bleed passage which is always open across the piston. This bleed orifice can be created by utilizing orifice notches positioned either on the flexible disc adjacent to the sealing land or by utilizing orifice notches directly in the sealing land itself. In order to obtain low speed control utilizing these open orifice notches, the orifice notches have to be small enough to create a restriction at relatively low velocities. When this is accomplished, the low speed fluid circuit of the valving system will only operate over a very small range in velocity. Therefore, the secondary or high speed stage valving is activated at a lower velocity that is desired. Activation of the high speed stage valving at relatively low velocities creates harshness because the shape of the fixed orifice bleed circuit force velocity characteristic is totally different than the shape of the high speed circuit.
High speed stage valving has been developed using valves on each pressure chamber side of the piston which operate upon different directional changes of the piston. Use of valves increases the cost and complexity of the shock absorber. Valves can also “chatter” during operation decreasing damping effectiveness and potentially increasing the potential for rapid wear of the valve parts.